A method of focusing particles is provided. The method includes transiting a fluid containing particles therein through a channel at a flow rate and adjusting the flow rate for a desired transit time through an interrogation zone through which a light from an excitation source passes. The method further includes optically exciting the particles with the excitation source, detecting an optical signal from the particles, and analyzing the optical signal. The particles may be droplets. Further, the particles may transit the interrogation zone in single file. A system of focusing particles is also provided. The system includes a channel having an inlet for accepting a fluid containing particles. The system further includes a flow adjuster configured to adjust the flow rate for a desired transit time through an interrogation zone, a light source configured to optically excite the particles, and a detector configured to detect optical signals from the particles.